The present invention relates generally to obtaining physiological parameters from animals or humans, and more particularly to a system for deriving respiratory related information from a blood pressure signal obtained from either humans or animals.
In animals or humans, it is often desirable to be able to monitor a variety of physiological parameters for purposes of research, therapeutics, and diagnosis. One such parameter that is of value is respiration. Simple measurement of the animal""s or human""s respiration rate have included the use of plethysmographs, strain gauges, chest impedance measurements, diaphragmatic EMG measurements, and other known measurement devices. For example in animal research, the plethysmograph approach requires the use of a non-compliant closed box in which the animal is placed with a means for measuring either the air flow in and out of the box or pressure changes inside the box or air flow at the animal""s mouth when breathing air from outside or inside the closed container. While plethysmographs work reasonably well with human subjects who can cooperate with test personnel, it is unreliable when dealing with laboratory animals, such as rats, dogs, monkeys, and etc.
The use of strain gauges and apparatus for measuring chest impedance changes generally require the animal to be tethered to the test equipment via electrical leads and the like. This does not lend itself to chronic testing and, moreover, strain gauges are quite sensitive to movement artifacts that can mask the desired signal output. Diaphragmatic EMG measurements can be used to determine respiratory rate, but electrode placement requires a higher skilled surgeon, and electrical noise from ECG and other sources can make accurate detection of respiration difficult.
To allow for animal mobility, it has proven advantageous to surgically implant sensors within the animal along with a telemetry transmitter so that the sensed signals can be electronically transmitted to an external receiver without the need for exteriorized conductive leads or catheters. U.S. Pat. No. 4,846,191 to Brockway, et al., owned by applicant""s assignee, describes an implantable blood pressure sensing and telemetering device suitable for long-term use in a variety of laboratory animals. A solid-state pressure sensor is fluid coupled to a selected blood vessel and the signal produced by the sensor is amplified, digitized and transmitted, transcutaneously, to an external receiver by means of a battery-powered transmitter. Once the blood pressure data are received, they are signal processed to recover features thereof, such as mean systolic pressure, mean diastolic pressure, mean arterial pressure, heart rate, etc. The Brockway et al. xe2x80x2191 patent also recognizes that the pressure sensing system used therein can be adapted to monitor intrathoracic pressure from which respiratory rate and other respiratory parameters can be derived. However, if it is desired to chronically monitor both blood pressure and respiratory activity following the teachings of the Brockway et al. patent, plural sensors and at least one telemetry transmitter with a multiplexing capability is required
The Kahn et al. U.S. Pat. No. 4,860,759 demonstrates the combined use of transthoracic impedance and strain gauge sensors to monitor respiration rate. The approach disclosed in the Kahn et al. patent suffers from many shortcomings, not the least of which is the quality of the resulting data.
An example of the use of multiple, chronically implanted sensors in laboratory animals is described in R. Rubini et al., Power Spectrum Analysis of Cardiovascular Variability Monitored By Telemetry in Conscious, Unrestrained Rats, Journal of the Autonomic Nervous System, vol. 45, at 181-190 (1993). In the experiments described in the Rubini et al. paper, telemetry equipment manufactured by Data Sciences International (applicant""s assignee) was utilized. However, the experimenters involved, while recognizing that a respiratory artifact was present in the sensed blood pressure data, discarded this component in favor of lower frequency components relating to sympathetic modulation of the cardiovascular system.
It is also known in the art that blood pressure is influenced by respiratory activity. In this regard, reference is made to H. Barthelmes and J. Eichmeier, A Device with Digital Display for the Determination of Respiratory Frequency from the Respiratory Fluctuations of Blood Pressure, Biomedizinische Technik, vol. 18, No. 4 (August 1973) and to D. Laude et. al., Effect of Breathing Pattern on Blood Pressure and Heart Rate Oscillations in Humans, Clinical and Experimental Pharmacology and Physiology, vol. 20, at 619-626 (1993). A system described in the Barthelmes paper employs an analog electronic filter to extract respiration rate from blood pressure signals. In the Laude article, human subjects were told to breath in rhythm with a metronome at several discrete frequencies while blood pressure was continuously measured. This study led to the conclusion that the relationship between systolic blood pressure and respiration differs from that between respiration and respiration sinus arrhythmia.
Although there are many studies in the published literature documenting variations of blood pressure with respiration, there is a need for an improved method and apparatus to derive respiratory parameters from blood pressure data.
The present invention provides a method and apparatus for obtaining physiologic parameter information of an animal or human other than blood pressure from a blood pressure signal indicative of sensed variations in blood pressure of the animal or human. The blood pressure signal is externally signal processed to develop an amplitude versus time waveform. A sequence of selected blood pressure features derived from individual cardiac cycles of the amplitude versus time waveform over a selected time interval are extracted from the developed amplitude versus time waveform. A mathematical model is fitted to the extracted sequence of selected blood pressure features to yield a fitted mathematical model. The physiologic parameter information is computed from the fitted mathematical model.
In a preferred embodiment of the present invention, the physiologic parameter information obtained is respiratory rate. In accordance with this preferred embodiment, a catheter tip of a blood pressure sensor is surgically implanted in an animal""s vascular system where the sensor provides an electrical signal related to variations in the animal""s blood pressure. A telemetry transmitter, also implanted in the animal, is connected to the blood pressure sensor for selectively transmitting a digitized version of the sensed electrical signal to an external receiver. The external receiver receives the digitized version of the sensed electrical signal and provides the blood pressure signal. The blood pressure signal is then signal processed in a digital computer to develop the amplitude versus time waveform, which is in a digitized form. The signal processing algorithm extracts from the digitized amplitude versus time waveform the sequence of selected blood pressure features such as beat-to-beat systolic data points, beat-to-beat diastolic data points, or beat-to-beat mean values of blood pressure over a predetermined time interval.
The mathematical model in this preferred embodiment is an nth order polynomial curve, which is preferably fitted to n+1 sequential points of the selected blood pressure features to obtain a fitted nth order polynomial curve. The fitted nth order polynomial curve preferably includes curve fit data values substantially equi-spaced in time. The curve fit data values are tested for critical points in accordance with a predetermined criteria such as a criteria relating peak values to zero-crossings of the fitted curve. Next, the computer determines from the critical points whether they are a maximum or a minimum whereby the respiratory rate can be determined from successive maximum and minimum critical points.
In one alternative embodiment of the present invention, spectral analysis is performed on the fitted mathematical model. In another alternative embodiment, several distinct blood pressure features for each cardiac Cycle are combined to yield a sequence of averaged blood pressure features, and the mathematical model is then fitted to the resulting sequence of averaged blood pressure features.
The implantable pressure sensor/transmitter described in the Brockway et al. xe2x80x2191 patent referenced in the background section provides an excellent means for monitoring blood pressure. By appropriately signal processing the telemetered blood pressure waveforms, respiration rate and other physiologic parameters can be derived therefrom according to the present invention.
Accordingly, the present invention provides an improved system for monitoring a plurality of physiologic parameters in animals or humans using a single implanted telemetric sensor. Moreover, the present invention provides a system for measuring blood pressure and respiratory parameters in such animals or humans using a single telemetric sensor. The present invention also provides a system for extracting respiratory related physiologic data from animals or humans using an implanted blood pressure sensor and telemetry transmitter where the transmitted blood pressure signal is externally signal processed to recover the respiratory information contained in the blood pressure signal. It is not, however, necessary that the blood pressure data be telemetered. A blood pressure sensor could be connected via wires to an amplifier and analog-to-digital converter and then input into the computer-based system according to the present invention.